The potential demand for ubiquitous wireless communications combined with restricted availability of the radio frequency spectrum has motivated intense research into bandwidth efficient multiple access schemes. A recent reference entitled "Spread Spectrum for Commercial Communications", by Schilling et al, as published in the IEEE Communications Magazine, Vol. 29, No. 4, April 1991 generally discusses various spread-spectrum techniques to effect multiple access communication and, in particular, one especially attractive avenue of approach, namely, Code-Division Multiple-Access (CDMA) techniques.
CDMA techniques take advantage of available bandwidth on the transmission medium, such as a fiber optic cable or the radio spectrum, by generating a set of pulses in the time domain which have appropriate correlation properties over predetermined time periods. Typically, the correlation property is such that a particular receiver tuned to a given transmitter code produces a detectable signal whenever the given transmitter code is presented to the receiver during each time period, whereas the output of the receiver is near zero for any other transmitter code presented to the receiver. A CDMA system operating on this time domain correlation property and utilizing a set of codes designated the optimal orthogonal codes was disclosed in U.S. Pat. No. 4,779,266; optimal orthogonal codes are but one type of more generic Direct-Sequence Spread-Spectrum (DS/SS) CDMA codes.
Demodulating a DS/SS CDMA signal in the presence of multiple-access interference has been previously addressed in the prior art. As alluded to above, the set of DS/SS waveforms assigned to different users are chosen to have small cross-correlations. This enables reliable communication of several DS/SS signals simultaneously over the same channel provided that all transmissions are received at approximately the same power. When there is a large disparity in received powers, however, the nonzero cross-correlations between the signals gives rise to the "near-far" problem, that is, a high-power transmission interferes significantly with the reception of a low-power transmission when a conventional matched filter receiver (or equivalently, a correlation receiver) is used.
The near-far problem can be mitigated, as taught in the prior art, by interference suppression schemes which use signal processing to exploit the structure of the multiple-access interference instead of treating it as noise. However, these schemes are significantly more complex than the matched filter receiver, and they require knowledge of the interfering signals. Representative of such techniques is the subject matter covered in the reference entitled "Near-far Resistance of Multiplier Detectors in Asynchronous Channels," IEEE Transactions on Communications, Vol. COM-38, No. 4, pp. 496-508, April 1990 as published by R. Lupas and S. Verdu. Hence, recent proposals for demodulation of DS/SS CDMA systems typically assume a matched filter receiver and deal with the near-far problem by controlling the power at the filter inputs, typically using feedback from the receiver. Such an arrangement is covered in the reference entitled "On the Capacity of a Cellular CDMA system," IEEE Transactions on Vehicular Technology, Vol. 40, No. 2, pp. 303-311, May, 1991, as published by K. S. Gilhousen, et al.
Symbol-by-symbol demodulation using the MMSE criterion can generally be implemented adaptively, when the parameters of the multiple-access interference are unknown. This eliminates one of the biggest drawbacks of interference suppression techniques proposed thus far, which typically require knowledge of the interfering signals. MMSE techniques have been used in equalization (as presented in the text Digital Communication, by E. A. Lee and D. G. Messerschmitt, published by Kluwer, 1988) and crosstalk suppression in wire channels (in a paper entitled "Suppression of Near- and Far-end Crosstalk by Linear Pre- and Post-filtering," IEEE Journal on Selected Areas in Communication, Vol. 10, No. 3, April, 1992). Also, it is noted that a related least-squares criterion has been previously proposed (in the article entitled "A Family of Suboptimum Detectors for Coherent Multiuser Communications," IEEE Journal of Selected Areas of Communication, Vol. 8, No. 4, pp. 683-690, May 1990) for sequence detection in the presence of multiple-access interference. The latter scheme is extremely complicated and hence is generally not amenable to continuous adaptation.
The art is devoid of teachings or suggestions of applying the MMSE technique to the demodulation of DS/SS CDMA signals.